The invention refers to a ship's hull with a cantilevered elastic stern tube arrangement for the elastic mounting of the tailshaft.
In order to eliminate propeller-induced vibrations of the ship's hull generated by hydrodynamic periodical forces it is known to apply elastic stern tubes, preferably cylindrical or conical and tapering towards the propeller, which surround the tailshaft and include at their aft ends the aft tailshaft bearing and the stern tube sealing.
According to the Patent Specification DE-PS No. 881 313 an elastic tailshaft mounting for vessels is known according to which a vibrating system within the hull is created mainly consisting of the propeller, the tailshaft and its two bearings, in order to minimise the propeller-induced vibrations. In this system the tailshaft and the shaft line are not coupled rigidly, but elastically, whereas the bearings of the tailshaft are not rigidly connected with the ship's hull, but via elastic elements.
According to the Patent Specification DE-PS No. 2 243 897 a cantilevered, double-walled, self-supporting stern tube arrangement for vessels for the elastic mounting of the tailshaft is known which is filled with lubricating oil and its natural vibration lying below excitation. In this arrangement the outer stern tube wall is seated at the thickened ends of the inner stern tube wall and surrounds the inner stern tube at a small distance without intermediate supports and is designed similarly to an airfoil following the flow along the ship's hull with fins tapering in flow direction, where the outer stern tube wall is shorter than the inner stern tube wall and the inner stern tube wall contains the lubricating oil filling and is connected with the outer stern tube wall of the ship's hull by means of screw connections enabling good natural vibration characteristics with a satisfactory section modulus of the clamping section and a relatively low overall weight of the stern tube arrangement. Additionally, the quantity of the lubricating oil required for the lubrication of the aft tailshaft bearing is reduced, because the inner stern tube wall contains the lubricating oil filling so that it is not necessary to fill the entire supporting stern tube up to the stern tube bulkhead with lubricating oil.
It is, however, a common characteristic of all stern tube arrangements that their stern tubes protrude self-supportingly in aft direction as an extension of the hull in way of the tailshaft and that their length normally exceeds the propeller diameter so that a vibrating system is created. The stern tube system is dimensioned in such a way that its natural frequency lies between the number of revolutions of the propeller and the pulse frequency (revolutions .times. number of blades).
As the self-supporting length of the stern tube is relatively large, the distance between the propeller and the stern frame is relatively large, too. This great distance between the propeller and the stern frame results in a relatively small wake, i.e. the difference between the ship's speed and the velocity of flow into the propeller. It is not possible to increase the wake or to influence it effectively in order to improve the propulsive efficiency or to reduce the engine power, respectively.